Many investigators agree that irregularities of cardiac rhythm diminish the efficacy of diastolic augmentation by means of intraaortic balloon counterpulsation (IABP) in patients with severe left ventricular impairment. However, reduction of IABP efficacy during arrhythmia has not been quantitated adequately. Consequently it has not been possible to ascertain either the maximal efficacy or optimally synchronized IABP during arrhythmia or the cost in hemodynamic benefits of widely used compromises in timing intended to promote patient safety during irregularities of cardiac rhythm. In turn, it has not been possible to evolve policies of clinical management based on objective criteria for patients with arrhythmias during AIBP. Data on the hemodynamic consequences of different IABP timing algorithms will be obtained in mongrel dogs in induced heart failure and complete heart block. A computer-controlled pacemaker will be used to simulate various patterns of ectopic beats, with and without IABP. The data will enable the quantitation of the hemodynamic effects of arrhythmias during IABP. They will also make possible the determination of whether IABP is likely to be ineffectual or even deleterious during certain arrhythmias, or whether optimal control over IABP timing has the potential of improving the clinical management of patients with arrhythmias.